1. Field of the Invention
The present invention relates to a resin composition that alters its solubility in an alkaline aqueous solution by exposure to infrared, and particularly, to a resin composition useful for an image recording layer of positive or negative-type planographic printing plate precursor capable of direct plate-making by scanning with an infrared laser beam based on a digital signal outputted from a computer or the like.
2. Description of the Related Art
Development in laser technology in recent years has been conspicuous, making it possible to easily acquire a solid laser or semiconductor laser having a light emitting region of, especially, from near infrared to infrared with a high output but in a small size. Especially, in the field of planographic printing, the lasers play a very important role as an exposure light source in direct plate-making based on digital data of a computer or the like.
A positive-type planographic printing plate material for infrared laser light in direct plating is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 7-285275. The invention in the publication relates to an image recording material obtained by adding a material absorbing light to generate heat and a positive photosensitive compound such as a quinonediazide compound to an alkali-soluble resin, wherein, in an image portion, the positive photosensitive compound works as a dissolution inhibitor substantially reducing a solubility of the resin soluble in an alkali aqueous solution, while in a non-image portion, the positive photosensitive compound is decomposed by heat so as not to exert a dissolution inhibiting capability and to thereby enabling the resin to be removed in development and an image is formed.
In such a positive-type planographic printing plate material, an example is described in JP-A No. 2000-241966, in which a phenolic hydroxyl group containing compound is added to an alkali-soluble resin for the purpose to improve a removability (solubility) of a non-image portion by an alkali developing solution. However, since this phenolic hydroxyl group containing compound also increases a solubility of an image portion, the technique is still at a level where further improvement is desired with respect to difficulty in attaining a sharp image contrast.
On the other hand, as a negative-type image forming method, there can be cited a recording method in which a condensation cross-linking reaction is caused by a heat treatment after exposure with an acid generated by light or heat as a catalyst to cure a recording layer of an exposed portion and an image portion forms. As a printing plate having such an acid-catalyst cross-linking type recording layer, a technique as described in JP-A No. 7-271029 has been known. Furthermore, a recording method can be cited in which a polymerization reaction is caused with a radical as an initiator generated by light or heat to cure a recording layer of an exposed portion and an image portion forms. As a printing plate having a recording layer polymerized by light or heat, there has been known a technique using a photopolymerizable or heat-polymerizable composition as a recording layer, which is described in JP-A Nos. 8-108621 and 9-34110.
Such a negative-type image recording material is lower in image forming capability compared with a positive type image recording material in which solubilization of a recording layer is caused by energy of infrared laser light irradiation; therefore, it is general to conduct a heat treatment prior to a developing process in order to accelerate a curing reaction through polymerization so as to form a rigid image portion.